It is well known to use SAW resonators for bandpass filtering in communications equipment, especially cellular radio equipment, to provide desirable characteristics such as low insertion loss (attenuation within the filter pass band), low ripple in the pass band, high stop band rejection (attenuation outside the pass band), small size, and low cost. Known SAW resonator filters include one-pole resonators, ladder circuit structure (LS) filters comprising a plurality of one-pole resonators, waveguide coupled double mode resonator (WCR) filters, and longitudinally coupled double mode resonator (LCR) filters. It is also known in such filters to provide a capacitance arrangement in order to provide the filter with sharper band edges, i.e. steeper transitions between the pass and stop bands of the filter.
For example, Hikita et al. U.S. Pat. No. 4,803,449 issued Feb. 7, 1989 and entitled xe2x80x9cFilter Combining Surface Acoustic Wave Resonatorsxe2x80x9d discloses a filter comprising a plurality of one-pole SAW resonators coupled together via capacitance elements. Mineyoshi U.S. Pat. No. 5,694,095 issued Dec. 2, 1997 and entitled xe2x80x9cSurface Acoustic Wave Resonance Device Adapted To Simple And Precise Adjustment Of Resonant Frequencyxe2x80x9d describes frequency adjustment of a single mode SAW resonator by providing an additional capacitance which may be in parallel with the resonator. Examples of LS filters are described in a paper by Ikata et al. entitled xe2x80x9cDevelopment Of Low-Loss Band-Pass Filters Using SAW Resonators For Portable Telephonesxe2x80x9d, Proceedings of the IEEE Ultrasonics Symposium, 1992, pages 111 to 115.
Double mode SAW resonators include WCR (also referred to as a transversely coupled resonator) and LCR (also referred to as an in-line coupled resonator) arrangements. For example, Xu et al. U.S. Pat. No. 5,821,834 issued Oct. 13, 1998 and entitled xe2x80x9cSurface Wave Device With Capacitancexe2x80x9d discloses a filter having two WCRs connected in cascade with one or more shunt capacitances to narrow the filter pass band. Other WCR filters with capacitance arrangements are disclosed for example by Gopani et al. U.S. Pat. No. 5,077,545 issued Dec. 31, 1991 and entitled xe2x80x9cSurface Acoustic Wave Waveguide-Coupled Resonator Notch Filterxe2x80x9d, and by Japanese Documents No. 4-230108 dated Aug. 19, 1992, No. 4-249907 dated Sep. 4, 1992, and No. 5-55855 dated Mar. 5, 1993, each in the name of Morii and each entitled xe2x80x9cSurface Acoustic Wave Filterxe2x80x9d.
In addition, Japanese Document No. 3-222512 dated Oct. 1, 1991, in the name of Komazaki et al. and entitled xe2x80x9cPolarized Type SAW Resonator Filterxe2x80x9d, discloses a plurality of one-pole resonators arranged transversely, or side by side, with a capacitance in parallel with the plural resonators to provide a steepened cut-off characteristic of the filter. WCR filters have a small fractional bandwidth (ratio of pass band width to center frequency of the pass band of the filter) suitable for example for IF (intermediate frequency) filtering in cellular communications systems. For filtering RF (radio frequency) signals in a cellular communications system, a much greater fractional bandwidth is typically required, for which it is known to use either LS filters as discussed above or LCR filters.
As is well known in the art, an LCR comprises two or more IDTs (interdigital transducers) arranged longitudinally, i.e. in line in the direction of SAW propagation, typically between two reflection gratings. LCR filters with capacitance arrangements are also known. For example, Nagaoka Japanese Document No. 2-130010 dated Nov. 10, 1988 and entitled xe2x80x9cSurface Acoustic Wave Devicexe2x80x9d discloses an LCR filter in which a capacitance, which may be constituted by parts of the IDTs, is arranged in parallel with an LCR to provide attenuation poles above and below the pass band of the filter, thereby providing steeper transitions between the pass and stop bands.
Similarly, Japanese Document No. 9-172342 dated Jun. 30, 1997, in the name of Honmo et al. and entitled xe2x80x9cDual Mode Surface Acoustic Wave Resonator Filterxe2x80x9d, discloses a filter having cascaded LCRs each with a parallel capacitance, connected between the input and output of the LCR, providing attenuation poles just above and just below the filter pass band, thereby steepening the transitions between the filter pass and stop bands. As shown clearly in FIG. 4 of Honmo et al., as the magnitude of the capacitance is increased, the attenuation poles are moved more closely to the center frequency of the filter pass band, the transitions between the filter pass and stop bands become more steep, and attenuation in the stop bands becomes substantially more degraded.
In addition, Sakamoto et al. U.S. Pat. No. 4,931,755 issued Jun. 5, 1990 and entitled xe2x80x9cSurface-Acoustic-Wave Device With A Capacitance Coupled Between Input And Outputxe2x80x9d discloses a SAW filter with multiple (e.g. five) IDTs coupled in line with one another, serving alternately as input and output IDTs, and a parallel capacitance connected between the input and output. The capacitance shifts an antiresonant frequency of the SAW device, on a high frequency side of the pass band, to a lower frequency closer to the center frequency of the pass band, thereby increasing close-in attenuation on the high frequency side of the pass band of the device. As shown in FIG. 3 of this reference, attenuation in the lower stop band, and far-out attenuation in the upper stop band, are substantially degraded.
Also, Shindo et al. Japanese Document No. 9-172350 dated Jun. 30, 1997 and entitled xe2x80x9cMultiple-Mode Surface Acoustic Wave Filterxe2x80x9d discloses a filter comprising two LCRs connected in cascade, with a capacitance divider at an output of the second LCR having its tapping point connected to a junction between the cascaded LCRs, to feed back part of the output signal and thereby provide a steeper attenuation characteristic of the filter. This is an IF filter for which only a small fractional bandwidth is required.
It is well known, for example from Sakamoto et al., Mineyoshi, and Xu et al. referred to above, that capacitances can be conveniently provided in a SAW device by capacitance between conductors on the substrate of the SAW device, for example in the form of an interdigital structure (IDS) on the substrate.
For filtering RF signals in current cellular communications systems, a fractional bandwidth of the order of 2 to 4% is typically required. The LCR filters of the prior art discussed above use the first (symmetric) and second (asymmetric) modes as the two modes of the LCR, and use piezoelectric substrates and orientations (crystal cuts and acoustic wave propagation directions) which do not provide filters with such a large fractional bandwidth. For example, the Honmo et al. reference discloses achieving a fractional bandwidth of about 0.3 to 0.5% using a substrate of 45xc2x0 X-Z (i.e. 45xc2x0 rotated X-cut Z-propagation) lithium tetraborate (LBO).
In contrast, for filtering RF signals with a large fractional bandwidth, it is desirable to use an LCR filter having a piezoelectric substrate and orientation that provides a high electromechanical coupling coefficient and a high SAW propagation velocity.
In order to address these desires, an article by Morita et al. entitled xe2x80x9cWideband Low Loss Double Mode SAW Filtersxe2x80x9d, Proceedings of the IEEE Ultrasonics Symposium, 1992, pages 95 to 104 discloses using LCR filters using the first (symmetric) and third (symmetric) modes with a substrate of 36xc2x0 Y-X lithium tantalate (LiTaO3) or 64xc2x0 Y-X lithium niobate (LiNbO3) providing high electromechanical coupling. This is a leaky SAW arrangement in which the acoustic waves are shallow bulk acoustic waves (SBAWs), specifically surface skimming bulk waves (SSBWs), and not Rayleigh wave SAWs. More particularly, there is no significant propagation of Rayleigh waves with these substrates. Conversely, in the other prior art discussed above the substrates are selected to minimize the propagation of bulk acoustic waves, including SBAWs, because these would detract from the performance of the filters using Rayleigh wave SAWs.
Distinctions between Rayleigh wave SAWs and SBAWs are known in the art and are discussed, for example, in xe2x80x9cSurface Acoustic Wave Devices And Their Signal Processing Applicationsxe2x80x9d by Colin Campbell, Academic Press, Inc., 1989. As explained therein (pages 11, 449-455), Rayleigh wave SAWs exhibit an elliptical displacement at the surface of the substrate, with components parallel to and normal to the surface. In contrast, SBAW devices propagate longitudinal bulk waves with shear horizontal (SH) polarization. The SH waves are excited and detected by the electric field parallel to surface-deposited IDTs in a similar manner (except that they are not excited by the electric field normal to the IDT fingers) to Rayleigh wave SAWs. In consequence, SBAW and Rayleigh wave SAW devices are generally indistinguishable visually; however, the devices are physically different because they have different substrate orientations propagating different types of acoustic waves. As further explained in this reference, bulk waves have higher velocities so that SBAW devices can be designed for operation at higher frequencies than Rayleigh wave SAW devices with similar geometry. SBAW devices can be grouped into two general categories, those with no energy trapping in which the SBAWs are referred to as SSBWs, and those with an energy trapping grating structure between input and output IDTs, in which the SBAWs are referred to as surface transverse waves (STWs).
As described in the article by Morita et al., the LCR comprises a center IDT and two lateral IDTs which are arranged symetrically and positioned longitudinally between two reflection gratings, so that the first and third longitudinal resonant modes, having a greater frequency separation than the first and second modes, are excited to provide an increased bandwidth of the LCR. As also described by Morita et al., this frequency separation of the first and third mode resonant frequencies is dependent upon a center-to-center finger spacing L of adjacent fingers of the center and lateral IDTs of the LCR. A spacing L=xcex/2 corresponds to a periodical positioning of the fingers throughout the center and lateral IDTs, but this can be increased or decreased. Morita et al. disclose that a maximum separation of the first and third mode resonant frequencies occurs when L=xcex/4 and again, with a phase shift through 180xc2x0, when L=3xcex/4, and maxima continue to occur with increasing spacings in steps of xcex/2, i.e. with L=nxcex/2xe2x88x92xcex/4, with the acoustic coupling being weakened and the bandwidth reducing as the integer n is increased. Thus Morita et al. recognize that the greatest bandwidth is provided with n=1 and L=xcex/4, for which there is no space between the adjacent fingers of the center and lateral IDTs (assuming a finger width of xcex/4) so that these must be grounded fingers.
Morita et al. also disclose characteristics of the resulting LCR filter, which as is well known in the art has a raised shoulder, providing a relatively poor attenuation, in the upper stop band on the high frequency side of and adjacent to the pass band. Typically, this raised shoulder may extend over a frequency band of up to about 75% of the bandwidth of the LCR filter, providing an attenuation of only about 12 dB compared with attenuations of the order of 20 dB or more in the lower stop band and in other parts of the upper stop band.
It should be appreciated that this raised shoulder is accepted in the art as an inherent but undesired characteristic of LCR filters of the type disclosed by Morita et al., and does not appear in and is not disclosed by the other prior art discussed above, because that prior art is concerned with different types of SAW filters, specifically Rayleigh wave SAW devices, rather than leaky SAW devices on substrates with high electromechanical coupling, propagating SBAWs generally and SSBWs specifically.
Illustrating this, Murai European Patent Application No. 0734121 published Sep. 25, 1996 and entitled xe2x80x9cSurface Acoustic Wave (SAW) Filterxe2x80x9d discloses in its FIGS. 1-3 prior art consistent with that of the Morita et al. article discussed above, and acknowledges the raised shoulder on the high frequency side of the pass band of the LCR, illustrating this at A in its FIG. 3. This reference describes various LCR arrangements using the first and fifth (symmetric) resonant modes with a 36xc2x0 Y-X lithium tantalate substrate (propagating SSBWs which are simply referred to as SAWs in the reference), with variation of a center-to-center spacing L of adjacent fingers of the input and output IDTs within a range from 5xcex/8 to 5xcex/4, and with and without series and/or shunt resonators to provide additional stop band attenuation at specific frequencies. In all of these cases, however, the raised shoulder on the high frequency side of the pass band of the LCR remains present and can be seen in the respective attenuation-frequency characteristics of the LCRs.
As evident from some of the prior art discussed above, it is also well known, in order to provide a sufficient attenuation in the stop bands, to connect together in cascade two or more 2-pole filters to provide a 4-pole, 6-pole, or higher-order filter, even though this results in increased insertion loss and complexity. Although such filters in a cascade are typically of the same type, this need not necessarily be the case. For example, as discussed above, Murai discloses additional resonators connected in series or shunt at the input and/or output of an LCR.
In addition, Taguchi et al. U.S. Pat. No. 5,592,135 issued Jan. 7, 1997 and entitled xe2x80x9cSurface Acoustic Wave Filter With Different Filter Portions Satisfying Complex Conjugate Relationship Of Impedancesxe2x80x9d discloses a cascade of an LCR filter with one or more LS filter sections constituted by series and shunt SAW resonators (also using a 36xc2x0 Y-X lithium tantalate substrate, propagating SSBWs which are again simply referred to as SAWs in the reference). Taguchi et al. illustrate typical characteristics of the cascaded sections in FIGS. 1 and 3, the latter illustrating the raised shoulder on the high frequency side of the pass band, this being compensated by the relatively good attenuation of the LS filter in this region. Conversely, the relatively poor far-out attenuation of the LS filter is compensated by the good attenuation of the LCR in these parts of the stop bands. Thus Taguchi et al. also illustrate the acceptance in the art of the raised shoulder on the high frequency side of the pass band in an LCR filter of the type disclosed by Morita et al.
Distinctions between Rayleigh wave SAWs and SBAWs or leaky SAWs, and LCR filters using the latter, are further known from xe2x80x9cSurface Acoustic Wave Devices For Mobile And Wireless Communicationsxe2x80x9d by Colin Campbell, Academic Press, Inc., 1998, especially Sections 13.2 to 13.4 on pages 360 to 373. FIGS. 13.3(a) and (b) on page 365 illustrate IDT structures for LSAW (leaky-SAW) LCR filters for operation in the first and second modes and in the first and third modes as discussed above. The same page states: xe2x80x9cThe two port structure of FIG. 13.3(a) looks deceptively like the two-port Rayleigh-wave resonators considered in Chapter 11, but its operation is significantly different.xe2x80x9d
With reference to FIG. 13.5 on page 367, illustrating the frequency response of a two-pole LSAW LCR with a 1 dB fractional bandwidth of about 3.6% and showing the raised shoulder on the high frequency side of the pass band as discussed above, this reference states on page 366: xe2x80x9cThe relatively poor cut-off response in the upper stopband is characteristic of this LSAW filter type, and may be attributed to high-frequency remnants of the distorted LSAW radiation conductance in FIG. 13.2.xe2x80x9d FIG. 13.2 on page 364 of the reference compares the radiation conductance and susceptance of transducers on substrates with standard Rayleigh wave SAWs and LSAWs, showing that the strong coupling of LSAW devices causes the radiation conductance to be distorted on the high frequency side, resulting in the raised shoulder. This is not the case for resonators using Rayleigh wave SAWs.
Japanese Documents No. 4-40705 dated Feb. 12, 1992 and No. 4-113711 dated Apr. 15, 1992, each in the name of Tada and each entitled xe2x80x9cLongitudinal Dual Mode Surface Acoustic Wave Filterxe2x80x9d, disclose two cascaded sections of energy confined resonators, on substrates of 36xc2x0 Y-X lithium tantalate and 41xc2x0 Y-X lithium niobate respectively, with a shunt capacitance connected between a junction of the cascaded resonators and ground to provide a filter with reduced spurious outputs in the pass band. These references are not concerned with improving attenuation in the stop bands of the filter.
An object of this invention is to provide an LCR using SBAWs and a piezoelectric substrate having relatively high electromechanical coupling, for example an LCR such as disclosed by Morita et al. as discussed above, in which the raised shoulder in the upper stop band attenuation-frequency characteristic, on the high frequency side of the pass band, is reduced or eliminated. As discussed above, none of the prior art addresses this problem, and instead the prior art simply accepts this raised shoulder as being inherent in the characteristic of such LCRs.
According to one aspect of this invention there is provided a filter having an input and an output, the filter comprising: a longitudinally coupled double mode resonator (LCR) comprising a plurality of interdigital transducers (IDTs), coupled between the input and the output, longitudinally coupled with one another on a surface of a piezoelectric substrate with an orientation for transducing and propagating shallow bulk acoustic waves (SBAWs), the LCR having a pass band and upper and lower stop bands and having a relatively poor attenuation in a raised shoulder region in the upper stop band on a high frequency side of the pass band; and a reactance coupled between the input and the output, the reactance being selected to couple a signal from the input to the output with a phase change generally opposite to a phase change of a signal coupled from the input to the output via the LCR at frequencies of said raised shoulder region, and with an attenuation such that the filter comprising the LCR and the reactance has a greater attenuation than the LCR at frequencies of said raised shoulder region.
Viewed alternatively, the invention provides a filter comprising a shallow bulk acoustic wave (SBAW) longitudinally coupled double mode resonator (LCR) and a reactance coupled between an input and an output of the resonator, the reactance and the resonator being arranged to couple a signal from the input to the output with generally opposite phases at frequencies in a shoulder region in an upper stop band on a high frequency side of a pass band of the resonator where the resonator has a relatively poor stop band attenuation, the reactance having a magnitude such that in said region attenuation of the filter comprising the resonator and the reactance is greater than attenuation of the resonator.
For use as an RF filter, preferably the resonator has a fractional bandwidth of about 2% to about 4%. To facilitate this, preferably the resonator comprises a center IDT between two lateral IDTs, with adjacent fingers of the center IDT and each lateral IDT having a center-to-center spacing of about xcex/4, where xcex is the wavelength of the propagated SBAWs at a center frequency of the pass band of the resonator. In this case the reactance preferably comprises a capacitance between conductors on the surface of the substrate, for example constituted by an interdigital structure on the surface of the substrate, typically with a magnitude of the order of 0.15 pF or less. The substrate is preferably selected from the group comprising, approximately, 36xc2x0 to 42xc2x0 Y-X lithium tantalate and 41xc2x0, 49xc2x0, and 64xc2x0 Y-X lithium niobate, providing high electro-mechanical coupling and a suitable orientation for transducing and propagating SBAWs rather than Rayleigh wave SAWS.
The invention also provides a filter comprising a plurality of filter sections connected in cascade, wherein at least one of the filter sections comprises a filter as recited above. One such filter can include two filter sections each comprising a filter as recited above, connected in cascade by connections of each of the two lateral IDTs of one filter section to a respective one of the two lateral IDTs of the other of the two filter sections, wherein there is no direct electrical connection between the two lateral IDTs of each filter section, and wherein the reactances of the two filter sections are connected to respective ones of said connections between the lateral IDTs of the filter sections so that the reactances do not provide an electrical path between an input and an output of the filter connected to the central IDTs of the two filter sections.
Another aspect of the invention provides a method of improving a relatively poor stop band attenuation in a shoulder region in an upper stop band on a high frequency side of a pass band of a longitudinally coupled double mode surface wave resonator comprising at least two interdigital transducers on a surface of a piezoelectric substrate with an orientation for transducing and propagating shallow bulk acoustic waves, comprising providing a reactance between an input and an output of the resonator, the reactance coupling a signal from the input to the output with a phase change that is generally opposite to a phase change of a signal at frequencies in said shoulder region coupled from the input to the output via the resonator, and with an attenuation such that in said shoulder region attenuation of the combination of the resonator and the reactance is greater than attenuation of the resonator.
A further aspect of the invention provides a filter comprising: a first longitudinally coupled double mode resonator (LCR) and a second LCR each comprising a center interdigital transducer (IDT) and two lateral IDTs longitudinally coupled with one another on a surface of a piezoelectric substrate for coupling acoustic waves between said IDTs; an input and an output of the filter connected to the center IDT of respectively the first and second LCRS; a first signal connection between a first one of the two lateral IDTs of the first LCR and a first one of the two lateral IDTs of the second LCR; a second signal connection between a second one of the two lateral IDTs of the first LCR and a second one of the two lateral IDTs of the second LCR, the second signal connection not being directly electrically connected to the first signal connection; a first reactance connected between the input of the filter and the first signal connection; and a second reactance connected between the output of the filter and the second signal connection.
Preferably each reactance comprises a capacitance constituted by an interdigital structure on the surface of the substrate, and preferably the substrate has an orientation for transducing and propagating shallow bulk acoustic waves (SBAWs), each LCR has a pass band and upper and lower stop bands and has a relatively poor attenuation in a shoulder region in the upper stop band on a high frequency side of the pass band, and each reactance is selected to increase attenuation in said shoulder region.